Disclaimer: Castle is not mine.
She goes to DC. She goes to DC and leaves him in New York, telling him that she loves him, that he's the best thing that's ever happened to her. He doesn't believe her.
She goes to DC. She texts him daily and they share pictures, phone calls, days on Skype. He tries to keep it up, tries to smile and love her whole although most of her is there, and not here. He breaks it off after six months, tells her it hurts too much, that he needs her in his arms and not just on the small screen of his phone. (She cries herself to sleep for a week.)
She goes to DC. They talk sometimes, not as often as they used to, though. Castle texts her when he sees something that reminds him of her, of them. When the conversation ends, she always tells him she loves him. He never says it back.
She goes to DC. Six months after Castle breaks it off with her, she returns to New York for a week, only waits to see him on her second to last day there. The minute she sees him she almost falls apart. She didn't tell him she was coming, and he's just as shocked as she expects. He's thinner. His face is almost hollowed out, and his chest is not the one she used to curl up on as his fingers skated up and down her spine. He hugs her, buries his face in her hair, his arms heavy around her waist. Tears threaten to fall and before they can make it past her eyelids, she tells him she needs to go. He follows her.
He stays in New York. He tells her he can't go with her to DC; that his whole life is here. His family, his work. This is where I grew up, Kate. For the first week he's bitter. Angry that she leaves, angry that she leaves him. He's mostly angry because he can't stop making two coffees, can't stop reaching to his left when he wakes up, expecting to wrap his arm around her waist and pull her against him. She is never there.
He stays in New York. He loves her, he'll always love her, but he can't keep doing this. He breaks it off after six months. He hates himself for it, considers it the biggest regret of his life just two days after it happens. He can't bring himself to call her and try to apologize, take it back. (He cries himself to sleep for a week.)
He stays in New York. He saves every picture she sends him, pastes it in a scrapbook he plans on giving her when he sees her again. Sometimes he sees things that remind him of her, but he doesn't send them. A story in the paper about an elephant at the local zoo giving birth, a scarf that he sees out shopping with Alexis that he thinks would go well with her eyes. She tells him she loves him whenever they talk. It hurts too much to return the sentiment. (He's never stopped feeling it.)
He stays in New York. Six months after he breaks it up, (one year after she goes to DC.) Paula decides he needs to get out, get some fresh air away from the thoughts of Kate that surround him in his loft. Paula's plan backfires when he sees Kate through the crowd of people and nearly runs to her. He hugs her tight, revels in being able to hold her again, doesn't care who sees them. She suggests leaving and he follows her to his loft. They make love, twice, and then twice after that. When he wakes the next morning, he reaches to his left and finds cold sheets. Her clothes are gone as well as his favorite white button up shirt, the one she wore after they spent their first night together, he notes. The only other thing that's missing is the scrapbook he's been making her, that was filled with the last two pictures she sent him of her at her local zoo, in front of the elephants.
She goes to DC.
He stays in New York.
A/N: So this is a very improbable thing to happen because come on, they wouldn't see each other once in six months? But I was in a bad mood, so you get a sad fic.